Nonaqueous electrolyte secondary cells of the type mentioned comprise a rolled-up electrode unit formed by laying a positive electrode and a negative electrode, each in the form of a strip, over each other in layers with a separator interposed therebetween and rolling up the resulting assembly into a spiral form. The rolled-up electrode unit is encased in a battery can.
The electric power generated by the rolled-up electrode unit is delivered to the outside through an arrangement including a plurality of conductive current collector tabs having their base ends attached to each of the positive electrode and the negative electrode of the electrode unit. The positive current collector tabs extending from the positive electrode have outer ends connected to a positive terminal portion, and the negative current collector tabs extending from the negative electrode have outer ends connected to a negative terminal portion. This arrangement is widely used.
However, the current collecting arrangement comprising a plurality of collector tabs has the problem of failing to achieve a sufficient current collecting effect when used in nonaqueous electrolyte secondary cells of large size having a high current value since the cell has increased electrode areas although producing a satisfactory current collecting effect in nonaqueous electrolyte secondary cells of small size which are relatively low in current value.
Further the connection of the current collector tabs to each electrode terminal portion requires a complex structure and complicated procedure, hence the problem of low work efficiency or productivity.
Accordingly, a cylindrical nonaqueous electrolyte secondary cell has been proposed which has a current collecting structure comprising a negative electrode current collector plate 36 and a positive electrode current collector plate 30 as shown in FIG. 7. This cell has a battery can 1 formed by a cylinder 15 and lids 16, 16 secured to opposite open ends of the cylinder. A rolled-up electrode unit 2 is enclosed in the battery can 1. The negative electrode collector plate 36 and the positive electrode collector plate 30 are arranged at respective ends of the electrode unit 2 and joined to the unit 2 by laser welding. The collector plates 36, 30 are connected by lead portions 37, 34 respectively to a negative terminal assembly 4 and a positive terminal assembly 40 mounted on lids 16, 16.
The rolled-up electrode unit 2 comprises a positive electrode 23, separator 22 and negative electrode 21 each in the form of a strip. The positive electrode 23 is formed by coating a current collector of aluminum foil with a positive electrode active material. The negative electrode 21 is formed by coating a current collector of copper foil with a negative electrode active material.
The positive electrode 23 and the negative electrode 21 are each superposed on the separator 22, as displaced from the separator widthwise thereof and rolled up into a spiral form, whereby the edge of the positive electrode 23 is positioned as projected outward beyond the edge of the separator 22 at one of opposite ends of the electrode unit 2 in the direction of its winding axis, and the edge of the negative electrode 21 is positioned as projected outward beyond the edge of the separator 22 at the other end of the unit 2. The positive electrode current collector plate 30 is made of aluminum, and the negative current collector plate 36 is made of copper.
With the current collecting structure wherein the collector plates 36, 30 are joined to the respective ends of the electrode unit 2 as described above, the collector plates can be welded to the unit 2 contactlessly without applying pressure to the plates for welding. This achieves an improved work efficiency or productivity.
The process for fabricating the nonaqueous electrolyte secondary cell shown in FIG. 7, however, has the problem that when the negative electrode collector plate 36 is disposed at and welded to the edge of the negative electrode 21 of the unit 2, sufficient energy can not be given to the portion to be welded since the copper forming the collector plate 36 has high reflectivity for the laser beam used for welding, forming a faulty weld and increasing the electric resistance between the unit 2 and the negative electrode collector plate 36 to result in an impaired current collecting efficiency. If the collector plate 36 is made from nickel, the weldability of the plate 36 to the electrode unit 2 can be improved, whereas the collector plate 36 of nickel has greater electric resistance than the plate 36 of copper and therefore exhibits a lower current collecting efficiency.
FIGS. 20 and 23 show another conventional nonaqueous electrolyte secondary cell, which comprises a cylindrical battery can 1 including a cylinder 15 and lids 16, 16 welded to respective opposite ends of the cylinder, and a rolled-up electrode unit 5 enclosed in the can 1. A pair of positive and negative terminal assemblies 110, 110 are mounted on the respective lids 16, 16 and each connected to the electrode unit 5 by a plurality of electrode tabs 6 for delivering the electric power generated by the unit 5 to the outside through the terminal assemblies 110, 110. Each lid 6 is provided with a gas vent valve 13 which is openable with pressure.
As shown in FIG. 22, the rolled-up electrode unit 5 comprises a positive electrode 51 and a negative electrode 52 each in the form of a strip and rolled up into a spiral form with a striplike separator 52 interposed between the electrodes. The positive electrode 51 is prepared by coating opposite surfaces of a striplike current collector 55 of aluminum foil with a positive electrode active material 54 comprising a lithium containing composite oxides. The negative electrode 53 is prepared by coating opposite surfaces of a striplike current collector 57 of copper foil with a negative electrode active material 56 containing a carbon material. The separator 52 is impregnated with a nonaqueous electrolyte.
The positive electrode 51 has an uncoated portion having no active material 54 applied thereto, and base ends of the electrode tabs 6 are joined to the uncoated portion. Similarly, the negative electrode 53 has an uncoated portion having no active material 56 applied thereto, and base ends of the electrode tabs 6 are joined to the uncoated portion.
With reference to FIG. 23, the electrode tabs 6 of the same polarity have outer ends 61 connected to one electrode terminal assembly 110. For the sake of convenience, FIG. 23 shows only some of the electrode tabs as connected at their outer ends to the terminal assembly 110, with the connection of the other tab outer ends to the assembly 110 omitted from the illustration.
The electrode terminal assembly 110 comprises an electrode terminal 111 extending through and attached to the lid 16 of the battery can 1. The electrode terminal 111 has a base end formed with a flange 112. The hole in the lid 16 for the terminal 111 to extend therethrough has an insulating packing 113 fitted therein to provide electrical insulation and a seal between the lid 16 and fastening members. The terminal 111 has a washer 114 fitted therearound from outside the lid 16, and a first nut 115 and a second nut 116 which are screwed thereon. The insulating packing 113 is clamped between the flange 112 of the terminal 111 and the washer 114 by tightening up the first nut 115 to produce an enhanced sealing effect. The outer ends 61 of the electrode tabs 6 are secured to the flange 112 of the terminal 111 by spot welding or ultrasonic welding.
Lithium ion secondary cells have the problem that an increase in the size thereof lengthens the positive and negative electrodes, consequently lowering the current collecting efficiency of the current collecting structure comprising electrode tabs to produce variations in internal resistance or result in a lower discharge capacity.
FIG. 21 shows a current collecting structure proposed to obtain a uniform current collecting efficiency over the entire lengths of the positive and negative electrodes. The proposed structure is provided for a rolled-up electrode unit 7, which comprises a positive electrode 71 prepared by coating a current collector 75 with a positive electrode active material 74, a negative electrode 73 formed by coating a current collector 77 with a negative electrode active material 76 and a separator 72 impregnated with a nonaqueous electrolyte. The positive electrode 71 and the negative electrode 73 are each superposed on the separator 72 as displaced widthwise of the separator, and rolled up into a spiral form, whereby the edge 78 of current collector 75 of the positive electrode 71 is positioned as projected outward beyond the edge of the separator 72 at one of opposite ends of the electrode unit 7 in the direction of its winding axis, and the edge 78 of current collector 77 of the negative electrode 73 is positioned as projected outward beyond the edge of the separator 72 at the other end of the unit 7.
A disklike current collector plate 62 is secured to each of opposite ends of the rolled-up electrode unit 7 by resistance welding and connected to the same electrode terminal assembly 110 as described above by a lead member 63.
The nonaqueous electrolyte secondary cell with the current collecting structure of FIG. 21, however, has the problem of being great in the internal resistance of the cell because the edges 78, 78 of the current collectors 75, 77 forming the positive electrode 71 and the negative electrode 73 of the electrode unit 7 have a small area, therefore providing a small area of contact between the collector plate 62 and each current collector edge.
It is especially required that lithium ion secondary cells, for example, for use as power sources in electric motor vehicles be of high capacity and reduced in internal resistance to the greatest possible extent so as to obtain a high power. Furthermore a current collecting structure of high productivity is required for a reduction of manufacturing cost.
Accordingly, a cell of low resistance and high productivity has been proposed which comprises a current collector plate having small bulging portions formed thereon as uniformly distributed over the entire surface thereof, such that the collector plate is secured to a current collector edge by resistance welding with the bulging portions in contact therewith to concentrate the current on the bulging portions and give improved weld strength (see, for example, JP-U No. 156365/1980).
As shown in FIG. 24, also proposed is a current collecting structure which comprises a current collector plate 92 prepared by forming a plurality of bent portions 94 on a flat platelike body 93, the bent portions 94 being secured to a current collector edge 78 of a rolled-up electrode unit 7 by resistance welding with the collector plate 92 pressed against the current collector edge 78 (see, for example, JP-A No. 31497/1999).
Further known are a current collector plate comprising two divided segments for suppressing ineffective current involved in attaching the collector plate by resistance welding to achieve an improved welding efficiency (JP-A No. 29564/1995), and a current collector plate having a projection V-shaped in section and formed on the portion thereof to be joined by resistance welding so as to concentrate the welding current on the projection and afford improved weld strength (JP-B No. 8417/1990).
Further proposed is a current collecting structure comprising a current collector member 95 in place of the disklike collector plate and formed with a plurality of slits 96 as seen in FIG. 25. For laser welding, a laser beam is projected onto the surface of the collector member 95 as disposed at an end of a rolled-up electrode unit 7, with a current collector edge 78 fitted in the slits 96 of the member 95 (JP-A No. 261441/1998).
Also proposed is a structure wherein a disklike current collector plate has a plurality of projections, V-shaped in section and up to 90° in end angle, and is welded to a group of electrode plates by irradiating the projections with a laser beam, with the collector plate pressed against each current collector (JP-B No. 4102/1990).
However, with the above-mentioned current collecting structure wherein the current collector plate is formed with small bulging portions as uniformly distributed over the entire surface thereof (JP-U No. 156365/1980), the collector plate is in unstable contact with the current collector, and the current fails to flow across these members depending on the state of contact, entailing the problem of producing a faulty weld.
The current collecting structure wherein the current collector plate has projections which are V-shaped in section or bent portions for the resistance welding of the plate (JP-A No. 31497/1999, No. 29564/1995 or JP-B No. 8417/1990) has the problem of low weld strength when the current collector has a very small thickness as is the case with lithium ion secondary cells.
The current collecting structure wherein the current collector member having a plurality of slits is secured to the current collector edge by laser welding (JP-A No. 261441/1998) not only requires the collector member which has a complex shape but also has the problem that the work of inserting the current collector edge into the slits of the collector member is very cumbersome.
With the structure wherein the disklike current collector plate having projections of V-shaped section is joined to the group of electrode plates by laser welding (JP-B No. 4102/1990), the projections have a V-shaped section of acute angle, so that the area of contact between the projection and the current collector edge is small, consequently entailing the problem of increased contact resistance. Since the junction between the V-shaped projection and the current collector edge is at an acute angle with the direction of projection of the laser beam for irradiating the junction, the laser beam fails to act effectively to weld the junction and is likely to produce a faulty weld.